Wideband Code Division Multiple Access (WCDMA) is a mobile radio access network standard specified by a 3rd Generation Partnership Project (3GPP) and used in third generation wireless data/tele-communication systems.
An example third generation communications system 100 is shown in FIG. 1. The system 100 includes a plurality of user equipment nodes (UEs) 110 that communicate with a Node B 120 through a radio air interface. The Node B 120 is controlled by a radio network controller (RNC) 130 and connected to a core network 140.
An enhanced uplink transmission capability from the UEs 110 to the Node B 120 was introduced in release 6 to 3GPP WCDMA. The enhanced uplink provides improved uplink packet-data support with reduced round trip delay, high bit-rate availability and increased cell capacity. As defined by 3GPP WCDMA release 6, during a Radio Resource Control (RRC) state CELL_DCH a network node allocates dedicated resources for each UE 110.
When a UE 110 transmits using the enhanced uplink it transmits data and control information on at least 3 physical channels Dedicated Physical Control CHannel (DPCCH), Enhanced-DPCCH (E-DPCCH), and Enhanced Dedicated Physical Data CHannel (E-DPDCH). The DPCCH transmits pilot bits that are known by the Node B 120 and also Layer 1 control information. The pilot bits are used as a reference by the Node B 120 to estimate the radio channel conditions (e.g. searcher, channel estimation, frequency offset estimation, and signal to interference ratio). The E-DPCCH transmits control information related to the enhanced dedicated physical data channel. The E-DPDCH transmits the data bits.
In 3GPP WCDMA release 8, support for enhanced uplink transmission in state CELL_FACH and IDLE_MODE was introduced. FIG. 2 illustrates two of the states, CELL-FACH configuration and CELL_DCH configuration, which the UEs 110 and Node B 120 can switch between for communications. In the states CELL_FACH and CELL_DCH a UE 110 can utilize common enhanced uplink resources that are setup by the network node for transmission of data on the E-DPDCH. As the number of smart phones increase in communication systems, the number of data transmissions of relatively small data packets will increase. These data packets may not be sufficiently small to be sent on the Random Access Channel which is used for data transmission in state CELL_FACH prior to 3GPP WCDMA release 8. The introduction of enhanced uplink in state CELL_FACH can decrease the load on the Random Access Channel.
Enhanced uplink in CELL_FACH may also provide a seamless RRC connection setup process through which a transition from the common enhanced uplink resource (state CELL_FACH) to a dedicated enhanced uplink resource (state CELL_DCH) takes place. In this way the RRC connection setup latency may be significantly reduced.
While a UE 110 transmits data on the E-DPDCH in state CELL_FACH, it utilizes a common network resource. When the UE 110 needs to make use of the resource for a longer time, the Radio Network Controller (RNC) 130 can switch the UE 110 from the state CELL_FACH to the state CELL_DCH. When the UE 110 switches, the common resource is released and the network assigns a dedicated resource to the UE 110. For the uplink layer 1 processing in the Node B 120, the switch from CELL_FACH to CELL_DCH may be a timing change, a change of uplink (UL) scrambling code, and possibly a change in the TTI. There can also be a change in the maximum data rate and hence a change in signal-to-interference ratio (SIR) target.
When the RNC 130 has decided to switch the UE 110 from the state CELL_FACH to the state CELL_DCH, referred to as an up-switch, it is desirable to do the transition as soon as possible. This will reduce the time that the UE 110 utilizes the common E-DCH resource. The transition can be made synchronous or asynchronous. For a synchronous up-switch, the up-switch takes place at a specified Connection Frame Number (CFN) that is decided by the RNC 130. In contrast, the asynchronous up-switch takes place as soon as the UE 110 can functionally carry out the state switch. An asynchronous up-switch can reduce the transition time significantly relative to a synchronous up-switch. Therefore, an asynchronous up-switch from the state CELL_FACH to the state CELL_DCH can be preferable.
In a WCDMA configuration of the system 110, the uplink and downlink are power controlled. The UE 110 signals to the Node B 120 how it shall regulate its downlink transmission power. In a similar way the Node B 120 signals to the UE 110 how it shall regulate its uplink transmission power. A new transmit power control (TPC) command is signaled every slot (1500 TPC commands per second).
The Node B 120 can control the UL packet error rate performance and UL interference by controlling the transmission power of the UE 110. As the UE 110 increases its transmission power the experienced signal to interference ratio in the Node B 120 will in general increase. An increased signal to interference ratio will result in a lower packet error rate. In this way the Node B 120 can tune the uplink packet error rate.
FIG. 3 illustrates graphs of transmission power levels and associated Transmission Power Control (TPC) commands that may be transmitted from the Node B 120 to a UE 110 through various dedicated physical channels and, vice versa, from the UE 110 to the Node B 120 to control the transmission power levels in the downlink and uplink directions. Referring to FIG. 3, the DPCCH is an UL physical channel which contains known pilot bits and Layer 1 control information. The Node B 120 measures the UL signal-to-interference ratio (SIR) on the DPCCH and compares it with a target value of the SIR. When the measured SIR is above the target SIR, the Node B 120 signals to the UE 110 to decrease its transmission power. When the measured SIR is below the target SIR, the Node B 120 signals to the UE 110 to increase its transmission power. For UEs 110 capable of transmitting enhanced uplink in CELL_FACH, the UL-TPC (Up Link Transmission Power Control) commands are signaled to the UE on the downlink (DL) channel F-DPCH (Fractional Dedicated Physical CHannel).
In a similar manner the UE 110 measures the quality of the F-DPCH that it receives from the Node B 120. When the quality is sufficient the UE 110 signals to the Node B 120 that it can decrease the transmission power on the F-DPCH. When the quality is not sufficient the UE 110 signals to the Node B 120 to increase the transmission power on the F-DPCH. The DL-TPC commands are sent to the network node on the uplink channel DPCCH.
The transmission power level of the E-DPCCH and the E-DPDCH may be controlled in response to a power offset relative to the transmission power level of the DPCCH.
US 2009181710 A1 describes how an initial power level is determined to transmit on the DPCCH. The initial power level is determined as the power used by the wireless transmit/receive unit in the CELL_FACH state prior to transitioning to the CELL-DCH state.
EP1487131 A1 describes a method in a network for mobile telecommunications of adjusting the transmission power of a Forward Access Channel (FACH) from a base station to a mobile user terminal. When a mobile user terminal transits from a CELL-FACH state to a CELL-DCH state, the initial transmission power to the mobile user terminal in the CELL-DCH state depends upon the last adjusted transmission power level in the preceding CELL-FACH state.
It is desirable to have a seamless, i.e. no loss of data at layer 1, transition from the state CELL_FACH to the state CELL_DCH. At the same time it is desirable to have an unsynchronized transition in order to obtain a fast state transition. From a downlink layer 1 perspective, the up-switch can correspond to a change in timing and possibly, channel code of the F-DPCH. From an uplink layer 1 perspective, the up-switch can correspond to a change of scrambling code and timing from a channel configuration of the first state to the scrambling code and timing of a channel configuration of the second state. The change in timing is the same in downlink and uplink.
In case of an asynchronous up-switch from the first state to the second state (i.e., from the state CELL_FACH to the state CELL_DCH), the new timing is known by the Node B 120 but the exact switch moment is not known by the Node B 120. The uplink layer 1 processing in the Node B 120 has to detect when the change of scrambling code and timing has taken place. The detection algorithm needs to perform the detection sufficiently fast in order not to lose any data that is transmitted on the E-DPDCH.
Since the Node B 120 does not know the exact switch time, it would be difficult or not possible to provide efficient transmit power regulation at the moment when the up-switch occurs. Too high of a UE 110 transmission power will increase the interference level in the communication cell provided by the Node B 120 which will have a negative effect on the cell throughput. In contrast, too low of a UE 110 transmission power will lead to an increased packet error rate and could result in a radio link failure.
In the DL, too high of a transmission power for the F-DPCH to one UE 110 will consume DL power resource, which could have been used for communication from the Node B 120 or another Node B to other UEs 110. The result could be a decreased cell throughput in the DL. In contrast, too low of a transmission power for the F-DPCH can cause an increased error rate for the UL-TPC commands, which may result in decreased performance in the UL.